1. Field of the Invention
The present invention relates to the reduction of oxides the nitrogen in the exit flue gas from the regeneration zone of a catalytic cracking operation employing carbon monoxide combustion promoters. More particularly, this invention is related to a method for controlling the amount of carbon monoxide combustion promoter in the regeneration zone to produce low levels of carbon monoxide and nitrogen oxides.
2. Description of the Prior Art
It has been found that oxides of nitrogen, primarily NO and NO.sub.2, are formed at high temperatures, such as the temperature at which catalyst utilized in a hydrocarbon cracking process is regenerated in the presence of carbon monoxide combustion promoters. As hydrocarbons such as petroleum feedstocks are cracked, coke is deposited on the catalyst particles. The coke formation on the particles progressively decreases the effectiveness of the particles. Eventually the effectiveness of the catalyst declines to the point where the coke must be burned off the particles. This step, which is normally referred to as regeneration, may be done on a batch or a continuous basis by contacting the catalyst particles with a regeneration gas, such as air. The conversion of the coke to CO and CO.sub.2 is an exothermic reaction in which a substantial amount of the liberated heat is absorbed by the catalyst. To utilize this heat in the cracking process, many cracking installations utilize a continuous circulation of catalyst between the cracking zone and the regeneration zone. To maximize the heat recovered from the regeneration zone, only slightly more than the stoichiometric amount of air required normally is utilized. Often, the combustion of the coke to CO.sub.2 in the regeneration zone dense phase catalyst bed is not complete, with combustion of the CO to CO.sub.2 also occurring in the dilute catalyst phase above the dense phase catalyst bed. This phenomena, which is known as "afterburning" may be injurious to the regeneration zone equipment and catalyst. Since there is relatively little catalyst in the dilute phase to absorb the heat of combustion, afterburning may raise the temperature in this region above the maximum permissible regeneration zone working temperature.
Increasingly stringent environmental regulations also restrict the maximum allowable concentration of CO in the regeneration zone flue gas. One method to minimize the amount of CO in the effluent gas and to prevent "afterburning" has been to increase the regeneration gas inlet rate. This method is not favored, however, since it decreases the amount of heat recovered and may require larger regeneration equipment, such as larger regeneration gas blowers. Accordingly, catalysts for promoting combustion or oxidation have been either incorporated into the cracking catalyst or separately added to the fluidized continuous cracking system to facilitate the conversion of CO to CO.sub.2 in the regeneration zone. It has been found that the concentration of the carbon monoxide combustion promoter must be kept relatively low, since the promoters contribute to the formation of excessive amounts of coke and hydrogen in the cracking zone.
Combustion promoters incorporating noble metals have been developed which effectively promote the conversion of CO to CO.sub.2 while not resulting in excessive coke and hydrogen formation. It has been found that utilization of these noble metal combustion promoters at the recommended levels of about 0.05 to about 100 parts per million by weight (wppm) of noble metal in the total weight of catalyst has led to the formation of significant quantities of oxides of nitrogen. Such oxides of nitrogen have several undesirable properties. Oxides of nitrogen are believed to contribute to eye irritation and to respiratory problems. In addition, oxides of nitrogen contribute to the formation of smog. As a result, air quality standards normally include a maximum allowable NO.sub.x concentration. For example, the national ambient air quality standard for the United States contains a maximum annual average NO.sub.2 concentration of one microgram per cubic meter, which is equivalent to about 0.05 parts per million by volume. While the oxides of nitrogen concentration may not be of immediate concern in many areas, in heavily populated locations increasingly stringent air pollution requirements may restrict the maximum allowable NO.sub.x concentration still further.